docs: def vertical_random_walk(size):
      """
      Generates random walks that start at the left side of the image and move towards the right
      :param size: The size of the image
      :return: An iterator of paths, each an iterator of (x,y) coordinates
      """
      width, _ = size
      distribution = {(dx, 1): 1 / 3.0 for dx in [-1, 0, 1]}
      start_points = [(x, 0) for x in range(width)]
      return random_walk_path(size, distribution, start_points)